In a mass spectrometer, an ion optical element called an “ion guide” is used in order to focus ions coming from the previous stage and send them into a mass analyzer, such as a quadrupole mass filter. A typical structure of the ion guide is a multi-pole structure having four, six or eight columnar (or tubular) rod electrodes aligned parallel to each other around an ion-beam axis. Normally, in these types of multi-pole ion guides, two radio-frequency voltages having the same amplitude with opposite phases are applied to the rod electrodes in such a manner that one radio-frequency voltage is applied to a pair of rod electrodes facing each other across the ion-beam axis while the other radio-frequency voltage is applied to another pair of rod electrodes which are adjacent to the former pair in the circumferential direction. By applying such radio-frequency voltages, a multi-pole radio-frequency electric field is created within a substantially columnar space surrounded by the rod electrodes. Ions are transported through this space while being oscillated due to the radio-frequency electric field.
In an ion guide described in Patent Literature 1, a set of virtual rod electrodes, each of which consists of a plurality of plate electrodes arranged along an ion-beam axis, is used in place of the normal rod electrodes. With the virtual-rod system, a DC electric field having a potential gradient along the ion-beam axis can be created so as to accelerate or decelerate ions, while making use of the excellent ion-focusing capability which is characteristic of multi-pole ion guides. It should be noted that the “multi-pole ion guide” in the present description includes such a “virtual” multi-pole ion guide using virtual rod electrodes.
In a liquid chromatograph mass spectrometer (LC/MS) or other mass spectrometers using an electrospray ion source or similar atmospheric pressure ion source, the configuration of a multi-stage differential pumping system is normally adopted so as to maintain a high degree of vacuum inside an analyzing chamber in which a mass analyzer and an ion detector are provided.
For example, in a mass spectrometer described in Patent Literature 2, three intermediate vacuum chambers are provided in tandem between an ionization chamber maintained at approximately atmospheric pressure and an analyzing chamber maintained in a high vacuum state, with the degree of vacuum being increased at each chamber from the ionization chamber to the analyzing chamber. To efficiently transportions in this multi-stage differential pumping system, a multi-pole ion guide is provided in each of the second and third intermediate vacuum chambers. Furthermore, an ion lens having an opening with a small diameter for allowing focused ions to pass through is provided in a partition which separates the second and third intermediate vacuum chambers.
The ion lens has the effect of focusing ions by a lens effect due to the DC electric field. However, a loss of ions occurs in a region near the boundary between the radio-frequency electric field created by the front-stage ion guide and the DC electric field created by the ion lens, as well as in a region near the boundary between the DC electric field created by the ion lens and the radio-frequency electric field created by the rear-stage ion guide, causing a decrease in the transmission efficiency of the ions. A probable reason for the loss of the ions is a disturbance of the electric field in the region near the boundary between the DC electric field and the radio-frequency electric field.
A mass spectrometer described in Patent Literature 3 has a multi-stage differential pumping system with an ion guide continuously extending over a length encompassing a plurality of intermediate vacuum chambers neighboring each other. In this system, since the radio-frequency electric field continuously extends through the plurality of intermediate vacuum chambers, the loss of the ions as observed in the system described in Patent Literature 2 does not occur, and a higher level of ion transmission efficiency can be achieved. However, an ion guide which extends over a length encompassing a plurality of intermediate vacuum chambers, i.e. which is provided in such a manner as to penetrate the partition walls separating the neighboring intermediate vacuum chambers, cannot be easily removed for the task of cleaning or replacement, and therefore, lowers the maintenance efficiency.